1. Field of the Invention
The present invention relates to small wattage electrical power needs such as ornamental light strings for decorating trees. In particular, the present invention relates to integrated circuits for rectifying and limiting current from an AC source.
2. Discussion of Background
Strings of individual, small wattage lights are widely used to decorate Christmas trees. Although these types of decorations are enjoyed by many people, they are sometimes a fire hazard, resulting in tragedies every Christmas holiday season. Most of these light strings draw a current of at least 200 ma at 120 VAC. Because of the concern over safety, these light strings may be made with heavier, gauge wire, such as 22 gauge rather than 24. However, even though a single string of lights may be hazardous, the common practice of cascading multiple strings of lights from one wall socket compounds this inherent danger associated with a single string.
In addition to Christmas tree light strings, there are other applications for electrical power for devices that have low power needs and especially those that use direct current (DC) rather than alternating current (AC), including many devices that use integrated circuits, such as calculators or devices that use light-emitting diodes. AC or large voltage surges can ruin these types of devices.
There have been some attempts to make Christmas tree decorations safer. Many of these involve putting fire or smoke detectors in ornaments. An example of this can be found in U.S. Pat. No. 4,075,614 issued to White and in U.S. Pat. No. 5,396,221 issued to Bridges. Fuses have been incorporated into the plugs of light strings, as taught for example by Cheng in U.S. Pat. No. 4,227,228 and Lehmann et al in U.S. Pat. No. 3,968,398. However, none of these address all of the problems with these types of light strings.
Smith et al, in U.S. Pat. No. 4,675,575, describe a light string based on light-emitting diodes (LEDs) which can be driven by pulsed DC power, which in turn is powered by household AC voltage. The pulsed DC is produced by a silicon controlled rectifier from the AC input.
DC is not unknown in connection with Christmas tree lighting using more conventional type incandescent bulbs. See Wu's stages selection type Christmas tree light controller circuit in U.S. Pat. No. 5,345,147, and Kramer's decorative lighting system in U.S. Pat. No. 3,789,211. However, in both cases the DC power is used only tangentially in achieving a particular lighting effect and not to power the bulbs themselves.
Turning briefly to the figures, FIGS. 1A, 1B and 1C illustrate graphs of voltages versus time, with voltage represented by the vertical coordinate and time represented by the horizontal coordinate.
FIG. 1A illustrates an alternating current (AC), meaning that the current reverses the direction of flow. So-called household current varies between approximately .+-.60 volts and does so at approximately 60 times per second. Direct current (DC) does not change direction, although it may change amplitude. FIGS. 1B and 1C both illustrate DC; FIG. 1B shows half-wave DC and FIG. 1C shows full wave DC. Both also illustrate pulsed DC.
It is known to generate half-wave and full-wave DC from AC by devices called converters or rectifiers. Devices for rectifying AC to DC are sometimes based on diodes, which are solid state circuit elements that conduct current in the forward direction but not in the reverse direction. For half-wave rectification, as illustrated in FIG. 1B, a single diode can exclude negative voltages, leaving only positive voltages. For full-wave rectification, two or four diodes are used.
Some of the patents identified above employ diodes in various functions in relation to power sources, such as disclosed by Smith et al. However, there remains a need for a better way to power low wattage devices and Christmas tree light strings in particular.